The flow of coolant (typically water) through the mold of plastics injection molding systems requires valves capable of rapidly controlling fluid flow over a wide temperature range. Injection molding is a well-known process which may be used for the fabrication of complexly-shaped plastic (or metal) parts. In the injection molding process, a molten plastic material is introduced into a mold and allowed to set or cure by cooling. Once the plastic is set or cured, the mold is opened, and the molded part is released. The temperature of the injection mold is preferably controlled so that the mold is at the proper temperature when the mold material is injected into the mold such that the object formed in the mold is set or cured at a rate that maintains the quality of the molded object while minimizing the setting or curing time to maximize production rates. Initially, an injection mold should be brought up to a steady-state operating temperature that is ideal for the particular molding operation. This can be achieved at start-up by, for example, forming a few scrap parts using heat from the liquid plastic to warm the mold or introducing heated water into channels within the mold. As hot molten material is injected into a mold, the mold absorbs heat from the molten material which must be removed from the mold to maintain the mold temperature within the ideal operating range. If heat is not removed at a sufficient rate, the mold temperature will tend to increase as a series of objects are successively molded. Mold temperature regulation is therefore generally desirable to maintain the temperature of an injection mold, both to minimize shrinkage and distortion during the setting or curing process and to ensure uniformity among a series of molded objects in a production run. Temperature control of an injection mold is typically accomplished by circulating cooling fluid through channels fashioned in the walls of the mold. The temperature of the mold initially increases upon the introduction of the hot molten material, but is restored to the desired operating temperature by the circulation of the cooling fluid through the channels in the mold. More precise control may be achieved through the use of multiple channels to circulate coolant through multiple zones in the mold.
Methods and devices for controlling the temperature of a fluid-cooled injection mold without the need for a continuous flow of cooling fluid are described in U.S. Pat. Nos. 4,354,812 and 4,420,446 to Horst K. Wieder. These patents describe methods by which an injection mold can be maintained at a desired operating temperature using a cooling fluid. Accurate control of the temperature of an injection mold can be achieved by mounting a temperature sensor onto or within the mold and using valves to control the flow of coolant based on the sensed mold temperature.
Water or petroleum based cooling fluids are commonly used for heat transfer in injection molding systems. A high-temperature injection molding process may involve normal mold temperatures in excess of 300° F., with molten material injected into the mold typically at a much higher temperature, e.g., 700° F., or higher. It is therefore necessary that the elements that carry the heat transfer fluids be capable of reliable service when exposed to fluids within these temperature ranges.
Controlling the flow of pressurized high-temperature fluids such as, for example, water, demands rugged valve construction. This is particularly true where long valve service life is required. Such valve design can be made even more demanding in applications in which the valve must also control large flow rates of fluids having wide temperature ranges. Such high-capacity valves may also be subject to the effects of water hammer when they close rapidly.
The operation of valves controlling the flow of pressurized heat transfer fluids will often lead to water hammer effects. Water hammer is a phenomenon related to the back pressure wave that results from an abrupt change in the flow rate of a fluid. The back pressure wave travels from the point where the flow was interrupted back towards the source of the flow. This can stress and damage hoses, pipes, joints, pumps and seals throughout the fluid system. Since the fluid is often pressurized within the system, leaks can result and lead to damage to equipment, controls, materials, and people in the area, in addition to causing costly down-time.
Water hammer is often dealt with by suppression measures that include such things as adding check valves or risers to limit or attenuate the back pressure wave, or to confine it to a particular area of the fluid system. However, these suppression devices, in addition to increasing cost, size, and weight, often introduce additional seals that must be maintained. Furthermore, over time, the gas in a riser dissolves into the fluid; consequently, the use of risers requires that the system be drained on a routine basis to maintain its water hammer suppression capability. Other mitigation techniques include increasing the pipe diameter to reduce the fluid flow velocity; however, such mitigation suppression techniques can add significant cost and require extra space.
Rapid valve closure can directly cause water hammer. Within a typical valve controlling a pressurized fluid flow, the plunger naturally tends to snap shut. As the plunger closes, fluid flow becomes restricted; but, before flow is entirely shut off, the velocity of the fluid around the plunger increases, causing a corresponding decrease in pressure that naturally leads to an accelerating closure force on the plunger. The resulting snap action tends to decrease the time it takes to interrupt the fluid flow, and it tends to produce a sharp step-like reflected pressure wave, i.e., water hammer.